This disclosure is related to the field of electrical power supply systems, in particular but not exclusively those used in electrically operated apparatus deployed in wellbores.
In some circumstances, such as in subsurface wellbore drilling using drilling tools having electrically operated devices therein, there is a need to generate electrical power proximate the drilling tools. Electrical power may be generated using a turbine rotated by the flow of drilling fluid through the drilling tools. The turbine may be rotationally coupled to a generator. Because the fluid flow in a drilling operation may vary, depending on, for example, the needs for washing out the drilling debris, as well as other functions provided by the flow of drilling fluid, and because the turbine and consequently the generator rotation speed varies with the change in flow, the generator rotational speed or RPM may vary over a relatively wide range. This variance is a problem because most electronic device power supplies have a limited usable voltage range.
Devices and methods known in the art for taking into account the expected range of fluid flow rates includes limiting the flow rate variance to a predetermined range and selecting a generator based on the RPM range that is expected. Different generators may be designed to operate in different RPM ranges. It known in the art for a drilling tool generator to be changed during a drilling operation because of the need to change the flow rate of the mud as the drilling progresses. Thus, an expected RPM range laterin a drilling operation may be different than the range which is supported by the generator used at the beginning of the drilling operation.
Attempts to address RPM range electrically without the need to change the generator have included switching relays to change generator output connection. Relay switching may cause a momentary interruption of the output power, which is typically undesirable for the circuitry being powered in the drilling tools.